(a) Field of the Invention
This invention relates to a semiconductor substrate adapted for the manufacture of a semiconductor integrated circuit such as the LSI and VLSI types, and, more particularly, to a silicon semiconductor substrate.
(b) Description of the Prior Art
Hitherto, the LSI type silicon substrate (wafer) has been provided by a wafer cut out of a single crystal silicon ingot grown by the Czochralski (abbreviated as "CZ") method. However, the ordinary CZ method has the following drawback. Since a quartz crucible is applied, the quartz is melted into a molten mass of silicon. As a result, oxygen is carried into a single crystal silicon ingot to the extent of 1.0.times.10.sup.18 cm.sup.-3. Later, the semiconductor substrate cut out of said single crystal silicon ingot is subjected to heat treatment at a temperature of about 900.degree. to 1000.degree. C., applied in the LSI manufacturing process. On this occasion SiO.sub.2 is crystallized out of the substrate because the released oxygen is contained in the substrate in the supersaturated form, leading to the occurrence of defects such as dislocation and stacking fault. When said dislocation or stacking fault is carried into the activated region of the substrate element, drawbacks such as PN junction leak, or, in the case of MOS memory, shortening of the pause time, are caused to occur.
To avoid the above-mentioned harmful effects caused by supersaturated oxygen, an intrinsic gettering (IG) wafer (hereinafter referred to as the "IG wafer") has been proposed. This wafer is prepared by letting the oxygen content of the activated element region escape to the outside by applying high temperature heat treatment and by depositing crystallized substances (fine defects) capable of gettering harmful metals, in the interior of said wafer. The IG wafer can, indeed, withstand the occurrence of dislocation or stacking fault near its surface. When applied to the manufacture of MOSLSI, however, the IG wafer cannot completely eliminate the defects of a thin oxidized membrane (gate oxidized membrane). The reason being that extremely fine defects retained in the single crystal silicon ingot are carried into the aforesaid oxidized membrane, leading to a decline in the withstand voltage property of said oxidized membrane. This drawback arises from the fact that since the extremely fine defects are carried into the silicon ingot not only by the oxygen contained in said silicon but also as a result of the instability of the solid-liquid interface occurring during the growth of the silicon crystal, they cannot be fully eliminated merely by letting said defects be released to the outside.
For the resolution of the above-mentioned problem, therefore, the Czochralski method, involving the concurrent application of a magnetic field, (abbreviated as "MCZ method") has been developed. This MCZ method can restrict the convection of a molten mass of silicon by the application of a magnetic field, thereby stabilizing the solid-liquid interface and controlling the oxygen concentration in the single crystal silicon ingot. Therefore, it is possible to ensure the growth of a single crystal silicon ingot containing less than 7.times.10.sup.17 cm.sup.-3. As well, in the heat treatment involved in the manufacture of LSI the above-mentioned MCZ method prevents SiO.sub.2 from being crystallized out, thereby preventing the occurrence of dislocation and stacking fault. Nevertheless, said MCZ wafer fails to obtain the intrinsic gettering effect which is generally ensured by the semiconductor substrate cut out of the single crystal silicon ingot grown by the ordinary CZ method. The MCZ method has a very weak resistance to contamination occurring during the manufacture of LSI; presenting difficulties in the formation of a highly stable element. When heat treatment of 1000.degree. C. is applied to a wafer cut out of a silicon ingot grown by the above-mentioned MCZ method, extremely fine defects (represented by black circles is FIG. 1) are produced in a region of low oxygen concentration (less than 6.times.10.sup.17 cm.sup.-3), though oxidized crystallized masses and stacking faults, illustrated by blank squares in FIG. 1, do not appear. The causes of the appearance of the above-mentioned extremely fine dot-like defects are not yet clearly defined. At any rate, this phenomenon is peculiar to the low-oxygen silicon crystal provided by the MCZ method.